The present invention relates to a total-reflection fluorescence apparatus with a doubly-curved crystal optic to improve the detection limit for ultra-trace analysis of contaminants and impurities on a surface.
Total-reflection x-ray fluorescence (TXRF) is a surface analytical technique for elemental analysis of particles, residues and impurities on a smooth surface. In particular, the TXRF method provides an effective means for detecting materials on surfaces that are of different composition than the composition of the surface. In this method, x-rays are directed onto a surface, typically an optically-reflective surface, with a grazing incident angle smaller than the total-reflection critical angle and are essentially totally reflected. Since the x-ray photons are totally reflected, very little x-ray photons are absorbed and scattered by the reflection medium. In contrast, foreign matter, such as particles, impurities, or contaminants, on the surface can absorb x-ray photons and produce characteristic secondary fluorescence x-rays which can be detected. Since little scattering and absorption by the reflection surface occurs, the fluorescence spectrum from the surface material itself is very low and little or no undesirable background fluorescent x-rays from the surface material is typically present. This results in very high sensitivity for measuring ultra-trace elements in, on, or near the surface of the reflection medium. This superior surface detectability makes the TXRF technique an important analytical tool for detecting foreign matter on surfaces, for example, for surface contamination control in semiconductor chip manufacture.
The rapid advance of semiconductor technology continues to demand lower detection limits for wafer surface contamination control. The detectability of TXRF apparatus based on the prior art described has approached its instrumentation limits and is unlikely to meet the demand of the semiconductor industry without significant improvement. One of the limitations of the prior art, for example, U.S. Pat. No. 5,249,216, is that only a very small fraction of the x-ray photons from the x-ray source which impinge upon the desired reflection surface contribute to the detection and measurement of the flourescent x-rays. In the prior art systems, such as the system disclosed in U.S. Pat. No. 5,246,216, most of the x-ray photons generated by the x-ray source are lost due to the poor collection capability of the optical elements, such as an aperture and monocbromator.
There are two types of monochromators used in the prior art, namely, multi-layer x-ray mirrors and crystal monochromators. Multi-layer mirrors have merit for low and medium energy x-rays due to their large d-spacing nature, that is, their large spacing between atomic planes. Multi-layer mirrors can be singly-curved in the dispersive plane and have laterally graded d-spacings thereby achieving intensity gain in the dispersive plane. However, in the direction perpendicular to the dispersive plane, multi-layer mirrors do not provide optical enhancement and the x-ray intensity suffers loss over the distance between the optic and the surface under examination. For example, for a multi-layer mirror designed for 9.7 keV x-rays (W Lxcex2, line), the effective capture angle is a few tenths of a degree in the dispersive plane and about 1 degree in the transverse direction. For high energy x-ray photons, for example, 17 keV or 22 keV (Mo Kxcex1 or Ag Kxcex1, curved multi-layer mirrors do not yield satisfactory results and a flat multilayer or a flat crystal monochromator is used without providing enhancement of x-ray intensity.
The present invention addresses the limitations of the prior art and provides an effective means for focusing x-rays upon a surface so that a more effective means of detecting the presence of undesirable foreign matter is provided. In general, the present invention, provides a method and apparatus which greatly improves the effective collection solid angle of the x-ray photons from an x-ray source and directs the x-ray photons to the surface under examination. In one embodiment, this is achieved by using an innovative doubly-curved optical device, for example, one of the optics disclosed in copending U.S. patent applications Ser. No. 09/342,606 filed on Jun. 29, 1999 and Ser. No. 09/450,323, filed on Nov. 29, 1999 (the disclosures of which are totally included by reference herein). In contrast to the prior art, a doubly-curve optic not only functions as a monochromator, but also functions as a strong x-ray focuser or concentrator to increase the x-ray flux upon the surface under examination.
One embodiment of the present invention is a total-reflection x-ray fluorescence apparatus comprising an x-ray source, an x-ray optical element having atomic planes curved both in the dispersive plane and in the transverse direction, a surface onto which the x-rays are focused to effect total x-ray reflection, and an x-ray detector to detect fluorescence signals of foreign matter on, in or near the surface. The foreign matter typically comprises particles, impurities, surface contaminants, or irregular layers and other matter that absorb and diffusely scatter the primary photons in the path of incident x-rays. The doubly-curved optic device captures a wide angle of x-ray photons from the x-ray source and through diffraction forms a monochromatic fan beam that has a large convergent angle in the direction perpendicular to the dispersive plane. In the dispersive plane, the convergent angle of the fan beam is limited to an angle less than the critical angle of the reflection interface. This convergent angle typically ranges between about 0.01 to 0.20 degrees. This convergent fan beam impinges on the essentially flat optical surface with an incident angle less than the critical angle and undergoes essentially total reflection. The foreign matter that is in the path of the x-ray beam absorbs the x-ray photons and emits secondary fluorescence x-rays that can be detected by an x-ray detector. Note that xe2x80x9ctotal-reflectionxe2x80x9d is a term of the art and implies that x-rays incident upon a surface arc essentially completely reflected without being absorbed or scattered by the surface. However, it is to be understood by those of skill in the art that, according to the present invention, some of the incident x-rays may not be totally-reflected from the surface but may be absorbed or scattered by any foreign matter present on the surface or by the surface itself.
The present invention also includes a doubly-curved optic having at least one set of atomic planes for diffracting x-rays ray photons which has a radius that is a function of the focal circle radius and the orientation of the atomic planes. Specifically, according to another embodiment of the invention, the radius, RP, of at least one of the atomic planes of the optic is a function of the radius, R, of the focal circle and the angle of orientation of the atomic planes, xcex1, relative to the surface of the optic, as expressed
RP=2R cos xcex1.xe2x80x83xe2x80x83Equation 1
The angle xcex1 typically ranging from 0 to 20 degrees. This geometry of the atomic planes provides for improved intensity of the diffracted x-ray photons upon the surface under examination compared to prior art atomic plane geometries. The atomic planes of this embodiment of the present invention are preferably doubly-curved to form a toroidal, ellipsoidal, spherical, paraboloid, hyperboloid, or any other type of doubly-curve shape. The present invention typically exhibits xe2x80x9casymmetric Bragg diffractionxe2x80x9d,that is, an optic according to this aspect of the invention has a source location and an image location that are not symmetric about the optic.
These and other embodiments of this invention will become more apparent upon review of the following drawings and the attached claims.